In wireless communications systems, communications devices may include a smart antenna that generates a plurality of directional antenna beams. Traditional methods for steering the directional antenna beams when operating in a wireless local area network (WLAN) are based on measuring raw signal strength that is received at the physical layer (PHY). Typically, the communications device attempts to steer the directional antenna beams to the direction where, for a certain measurement time interval, the average received signal strength is maximized.
A disadvantage is that the communications device blindly maximizes the received signal strength without identifying the source of the signals. In a more intelligent scheme, the communications device listens to the broadcast signals transmitted either periodically or quasi-periodically by the central access coordinator of the network, i.e., an access point (AP) or base station. The broadcast signals include beacon frames, for example.
The communications device steers a directional antenna beam to the direction where the strength of the signal received from the beacon frames is maximized. In this case, the station relies on the periodicity information regarding the time of transmission of the broadcast beacon frames from the access points. The system also measures the received signal strengths of the beacon frames for different directional antenna beams. This measurement is performed by scanning the signal strength of each beam periodically to lock onto the beacon frame using knowledge of the periodic transmission time of the beacon frames from the access point.
There are problems with antenna beam steering methods based on medium access control (MAC) agnostic maximization of the received signal quality, such as the received signal strength indicator (RSSI) or the signal-to-noise ratio (SNR). It is typically difficult to measure PHY signal quality indicators accurately, and especially if the quality indicator of the signal-to-noise or interference ratio is poor. This is due to the fact that it is difficult at the receiver to know whether the currently received signal includes an undistorted signals plus random noise, or if the received signal itself is distorted and directional interference is also added in the received signal.
Even in more intelligent antenna beam steering methods based on listening to broadcast signals such as the beacon frame signals that are known to be transmitted from the access point at periodic or quasi-periodic time intervals, there are still problems. The period is either known or can be estimated, and the different antenna beams can be steered for measuring received beacon frame signal strength at different measurement intervals and then compared.
One of the problems is that the timing of the beacon frame signals is not known precisely since beacon frames are only quasi-periodic, and can be lost due to either poor reception environments or loss of the exact timing of the transmission of WLAN signals. Another problem with the current systems is the relative long time delay, typically on the order of 100 msec, which the communications device needs to wait to receive one beacon frame packet in a WLAN.
Since no more than one beacon frame packet can be received per antenna beam every 100 msec, and since reception of a number of packets is needed per antenna beam to ensure reliable measurement of signal strengths, it can take a long time, on the order of seconds or longer, for a sweeping search of “best antenna beams” to be performed if the beam search is purely relying on reception of beacon frames. For some applications where fast beam searching is necessary because the application cannot tolerate possible degradation in communications during a beam search (including beams that would result in poor communication), such long delays incurred by steering methods relying on periodic reception of packetized broadcast signals can be a significant problem.
For example, a problem arises when measurements of WLAN signals are made without intelligent use of the received MAC packet information. When MAC packet-wise information is not used, it becomes very difficult for the wireless LAN station to distinguish the preferred antenna beam directions as well as undesired antenna beam directions.